In recent years, WDM transmission systems, which utilize Wavelength Division Multiplexing (WDM), have been used widely. WDM can transmit a plurality of optical signals of different wavelengths. In a WDM transmission system, each node is provided with a Reconfigurable Optical Add Drop Multiplexer (ROADM). An ROADM can drop an optical signal of a desired wavelength from a WDM optical signal and also can add an optical signal to an unused channel in a WDM optical signal.
In a WDM transmission system, each node is provided with an optical amplifier in order to compensate for losses in transmission path fibers and ROADMs. As an optical amplifier for amplifying a WDM optical signal, an Erbium Doped Fiber Amplifier (EDFA) for example is used.
The wavelength characteristics of the optical gain and the optical loss regarding a WDM optical signal depend upon the wavelength allocation of the WDM optical signal. Accordingly, a WDM transmission system has a function of adjusting the optical power of each wavelength channel based on the wavelength allocation of a WDM optical signal. This function is implemented by for example an optical channel monitor (OCM), which detects the optical power of each wavelength channel, and a wavelength selective switch (WSS), which adjusts the optical power of each wavelength channel. In such a case, the optical power of each wavelength channel is controlled so that the powers of optical signals arriving at a receiving node are maintained within the receivable power range of the optical receiver.
However, it takes several hundreds of milliseconds through several seconds for an optical channel monitor to detect an optical power and/or for a wavelength selective switch to adjust the optical power. This may lead to a situation where a change in the wavelength allocation of a WDM optical signal results in temporary but large variation in the optical power of each wavelength channel. When the power of an optical signal arriving at a receiving node gets out of the receivable power range of the optical receiver, a signal error occurs.
This problem arises when a change in the wavelength allocation of a WDM optical signal causes a large change in the optical gain wavelength characteristic. In a case where gain ripples for a WDM optical signal are large in a steady state, a change in the optical gain wavelength characteristic caused by a change in the wavelength allocation of a WDM optical signal is large. In other words, if gain ripples for a WDM optical signal in a steady state can be reduced, a change in the optical gain wavelength characteristic caused by a change in the wavelength allocation of the WDM optical signal may be reduced. In such a case, even when the wavelength allocation of a WDM optical signal changes, variation in the optical power of each wavelength channel is suppressed, leading to suppression of signal errors in an optical receiver. Note that a gain ripple refers to variation in the optical gain for a wavelength.
Spectral Hole Burning (SHB) is one factor that causes a gain ripple of an EDFA. Spectral hole burning occurs when an optical signal passes through an EDFA. Specifically, when an optical signal passes through an EDFA, the gain at a wavelength of the optical signal and its adjacent wavelengths is decreased.
Thus, reduction in a gain ripple caused by spectral hole burning suppresses variation in the optical power of each wavelength channel even when the wavelength allocation of the WDM optical signal changes.
As a configuration for suppressing variation in a gain ripple, an optical transmission device that adds pseudo light to a wavelength band not used in an optical signal is proposed (Japanese Laid-open Patent Publication No. 2008-091995 for example). Also, as a related art, a method of measuring accurately the intensity of an optical signal in a WDM communication system is proposed (Japanese Laid-open Patent Publication No. 2008-139073 for example). Further, the following documents describe the modeling of an EDFA and spectral hole burning.    C. Randy Giles and Emmanuel Desurvire, Modeling Erbium-Doped Fiber Amplifiers, Journal of Lightwave Technology, Vol. 9, No. 2. 271-283 (1991)    Maxim Bolshtyansky, Spectral Hole Burning in Erbium-Doped Fiber Amplifiers, Journal of Lightwave Technology, Vol. 21, No. 4. 1032-1038 (2003)
According to conventional techniques, it is difficult to suppress efficiently a gain ripple caused by spectral hole burning. This also makes it difficult to suppress a change in a gain ripple accompanying a change in the wavelength allocation of a WDM optical signal. Note that in the method that adds pseudo light to a wavelength band not used in an optical signal, some wavelength channels in the WDM optical signal are not able to be used for data transmission.